1. Field of the Invention
This invention relates to electric discharge lamps and has particular reference to an improved fluorescent lamp in which the mercury-vapor pressure during operation is regulated by means of an amalgam.
2. Description of the Prior Art
Low-pressure mercury-vapor discharge lamps that contain a strategically located body of a metal such as indium, cadmium or the like which forms an amalgam with mercury and regulates the mercury-vapor pressure within the lamp during operation are well known in the art. A fluorescent lamp of this type is described in U.S. Pat. No. 3,007,071 issued Oct. 31, 1961 to A. Lompe et al. Lamps regulated in this manner inherently operate in a "mercury-starved" unstable condition at a low light-output level for a certain period of time after they are first started since the amalgam reservoir heats up slowly because of its location within the lamp. The resulting slow "warm-up" problem under cold-start conditions has been corrected by employing an auxiliary source of amalgam that is located near one of the lamp electrodes and is thus rapidly heated and quickly releases mercury vapor. Fluorescent lamps having such auxiliary sources of amalgam placed on selected parts of the stem or mount structure are described in various patents such as U.S. Pat. No. 3,227,907 issued Jan. 4, 1966 to Bernier et al and U.S. Pat. No. 3,629,641 issued Dec. 21, 1971 to Kuhl et al.
Another problem encountered in fluorescent lamps, particularly those designed for operation at high power loadings, is that the lamps sometimes do not fail in a safe manner at the end of their useful lives. This occurs when the emission material on the electrodes becomes exhausted and the arc strikes the lead wires and causes them to melt or soften sufficiently that they contact the glass bulb and cause it to crack. As a safeguard against this potential hazard, the lamps are provided with an internal "fail-safe" structure that provides an electrically-conductive path from one or both lead wires to a portion of the glass stem. At the end of the useful life of the lamp the arc discharge is accordingly directed or drawn by the fail-safe conductor means to the stem -- thus cracking and puncturing the stem and rendering the lamp inoperative.
A fluorescent lamp wherein the fail-safe component comprises a wire or a coating of conductive material that is applied to the stem press and connected to one of the leads is disclosed in U.S. Pat. No. 3,265,917 issued Aug. 9, 1966 to J. G. Ray. A fluorescent lamp wherein the fail-safe conductive component comprises a strip of aluminum powder that is coated onto the stem press and contacts one of the lead wires is disclosed in Japanese Patent Publication No. 44-15840 dated July 14, 1969 of Sometani et al (applied for on May 12, 1965 by Toshiba Electric Company).
An amalgam-type fluorescent lamp wherein the dual functions of fast "warm-up" and fail-safe operation are achieved by means of a notched yoke of wire mesh or sheet metal that is "clipped" onto the stem press and carries an auxiliary source of amalgam is described in U.S. Pat. No. 3,562,571 issued Feb. 9, 1971 to Chalmers Morehead and the author of the present invention. While such metal-clip components provide the desired fast warm-up and fail-safe features, they are rather expensive from a material and lamp-manufacturing standpoint. A structure which performs both functions in the same reliable and positive manner but which is more economical would accordingly be very desirable and advantageous.